Pressure-sensitive recording sheets may be a combination of sheets which include an upper paper comprising a paper support having provided thereon a microcapsule layer containing microcapsules prepared by encapsulating oil drops of a substantially colorless electron donating color formers dissolved in an appropriate solvent, a lower paper comprising a paper support having provided thereon a developer layer containing an electron accepting developer and, in some cases, an intermediate paper comprising a paper support having provided on one side surface a microcapsule layer and on the other side surface a developer layer; a sheet comprising a paper support having provided on one surface both the above described capsule and a developer; and a sheet comprising a paper support wherein one of a capsule or a developer is contained therein and the other is coated thereon.
The above described sheets are disclosed, for example, in U.S. Pat. Nos. 2,505,470, 2,505,489, 2,550,471, 2,730,457 and 3,418,250.
However, the above described sheets do not always achieve satisfactory results. For example, when a microcapsule layer containing a color former is piled on an intermediate paper or a lower paper facing the developer layer thereof after printing is done on an upper paper or an intermediate paper, fog formation takes place on the developer layer with the passage of time. When an upper paper is used alone, as when the capsules are ruptured, color forms on the capsule layer and the capsule layer is thus stained.
The color stain of the capsule layer is remarkably heavy, particularly when styrene-butadiene latex is used as a binder for the capsule layer.